1. Field of the Invention
The present invention relates to a device for use in downhole operations. More specifically, the invention relates to adjusting the viscosity of a working fluid in a wellbore tractor to control vibration in the wellbore tractor.
2. Description of the Related Art
Coiled tubing and wire line may be used for deploying various downhole assemblies within a wellbore for performing various wellbore operations. The operations may be performed open hole before the well has been cased or lined, or after the well has been completed and having casing cemented within the wellbore. Example operations include setting or unsetting a tool within the wellbore, interrogating wellbore conditions such as by acoustics or resonance imaging, perforating within a wellbore, and the like. Increasingly, wellbores are drilled having lateral or deviated portions that are oriented oblique to a vertical axis of a primary wellbore. Wireline cannot be used for deploying tools in highly deviated wells, and coiled tubing is limited in its ability to urge the tools along these deviated portions. Moreover, coiled tubing can buckle and lockup to prevent movement of the tractor. Thus, tractor assemblies may be employed with the downhole tool for moving the tool through the deviated or lateral wellbore portions.
Typically, the tractors include a gripper portion that is selectively extended away from the downhole tool and into contact with an inner wall of the wellbore for pushing against the wall of the wellbore. The pushing by the gripper in turn motivates the downhole tool through the deviated or lateral section. Example grippers include wheels or rollers on the end of a gripper arm, or linkage assemblies that pivot out and push the tool along in an inchworm fashion. The tractor assemblies are often powered by a hydraulic system that is selectively pressurized for activating the grippers of the tractor assemblies.
Effectiveness of the tractor assemblies can be hampered by inconsistencies in the wellbore wall, either through changes in type of casing or, in an open hole condition, areas where the compressive strength of the formation varies. Washout sections in a wellbore can also introduce performance obstacles for wellbore tractors. To accommodate these inconsistencies, the tractor assembly must respond by altering the amount of extension away from the tool and/or the force supplied to a gripper arm and against a wellbore wall. The variations in applied force can introduce vibrations into the tractor assembly and the downhole tool that can be problematic for the movement of the downhole tool through the wellbore.